Electron discharge device of the velocity modulation type



Fe 2, 1949. J. H. FREMLIN 2,462,037

ELECTRON DISCHARGE DEVICE OF THE VELOCITY MODULATION TYPE Filed April 15, 1945 4 Sheets-Sheet l is v Attorneg Feb. 22, 1949. J. H. FREMLIN ELECTRON DISCHARGE DEVICE OF THE VELOCITY MODULATION TYPE 4 Sheets-Sheet 2 Filed April 15, 1945 Inuntor A WM I m 7 Feb. 22, 1949.

J. H. FREMLIN ELECTRON DISCHARGE DEVICE OF THE VELOCITY MODULATION TYPE Fi led April 15, 1945 4 Sheets-Sheet 3 CF/GG 5 K QWQQQQQQQQQM l \bhmh hbhhh I n uentor Irhn Heauor ham/m By I t Attorne J. H. FREMLIN ELECTRON DISCHARGE DEVICE OF Feb. 22, 1949.

- THE VELOCITY MODULATION TYPE Filed April 13, 1945 4 Sheets-Sheet 4 (JO/l X /A L L lNE RES ONA TOR PL A TES CATHODE' I CONTROL ELECTROD APERTURED PLATE- 5 I Invenior'; J0 Fm Heaver Frern Lin C. 4 f

l Horn y Patented Feb. 22, 1949 Nl'lED STATES ELECTRON DISCHARGE DEVICE OF THE VELOCITY MODULATION TYPE John Heaver Frcmlin, London, England, assignor,

by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a

corporation of Delaware Application April 13, 1945, Serial No. 588,207 In Great Britain April 19, 1943 Section 1, Public Law 690, August 8, 1946 Patent expires April 19, 1963 The present invention relates to electron discharges devices, and especially to those of the electron velocity modulation type, and is principally concerned with means for preventing the introduction of excessive noise which may occur under certain conditions of operation of these devices.

Velocity modulation devices may be operated by single transit, that is, so that each electron makes only one journey across the device and is then collected by an electrode; or they may be operated by reflecting the electrons back from an electrode after the first transit, so that they make at least another journey through the device in the reverse direction before being collected.

It has been found that when some types of velocity modulation devices employing magnetic focussing of the electron beam are used as local oscillators in high frequency receivers, serious amounts of noise at the intermediate frequency may be produced. We have established that this noise, which is of a random character and operates over a relatively wide frequency band, occurs when the device is operated by electron reflection but not in the case of a single transit operation. For example, in devices employing a co-axial line resonator of the kind described in U. S. Patent No. 2,320,860, an increase of receiver noise corresponding to about 30 decibels may occur when the operation is changed from single transit to electron reflection. The occurrence of this noise depends both upon the operating voltages and on the strength and direction of the focussing magnetic field. By very careful adjustment of these conditions it is usually possible to eliminate the noise, or to shift it to a frequency band where it is innocuous: but the necessity for such careful adjustment makes the use of the device impracticable except in highly skilled hands.

The principal object of the invention, therefore, is to provide a construction for an electron discharge device in which the cause of the excessive noise which is liable to occur with operation by electron reflection is partly or wholly removed so that the necessity for critical adjustments is avoided.

According to the invention, there is provided an electron discharge device comprising means for projecting a beam of electrons in one direction through a structure, and means for preventing electrons reflected back in the reverse direction through the structure from returning to the source of the beam.

According to another aspect, the invention 14 Claims.

provides an electron discharge device comprising means for projecting a beam of electrons through a passage in a resonator, and for reflecting them back again through the same passage, and means for preventing the reflected electrons from emerging from the passage at the end into which the beam is projected.

According to still another aspect, the invention consists in an electron discharge device in which a co-axial line resonator is excited by projecting a beam of electrons through the resonator, comprising means for reflecting the beam back'across the resonator in the reverse direction, and means for causing the resonator to collect the electrons as soon as they have completed one return journey across the resonator.

The invention may also provide an electron discharge device in which electrons are projected in a beam through a resonator and are reflected back again in the reverse direction through the resonator, comprising means for dividing the beam into a number .of separate pencils and means for collecting electrons returning in the spaces between the pencils whereby materially to reduce the number of to-and-fro journeys made by the electrons before collection.

The invention will be described with reference to the accompanying drawings in which:

Fig. l is a diagrammatic cross-section of an electron velocity modulation device of the coaxial line type to show the path of the electron beam through the resonator;

Fig. 2 is a diagrammatic vertical section of the device to indicate the path of a single electron passing backwards and forwards through the resonator.

Fig. 3 is a diagram similar to Fig. 2 of a device according to the invention to show the double transit path of a single electron;

Fig. 4 is a cross-section diagram like Fig. 1, but corresponding to Fig. 8;

Fig. 5 shows in exploded perspective form the elements of a device according to the invention; and

Fig. 6 shows a modification of Fig. 5.

Fig. 7 is a diagrammatic assembly View of the tube shown in Fig 5.

In Velocity modulation devices of the existing types operated by electron reflection, the electrons generally make a large number of journeys backwards and forwards across the resonator under the influence of the focusing magnetic field, and it is this circumstance which we have found to be the ultimate reason for the noise which is found to occur. Referring to Fig. 1,

the electron beam is projected from the cathode I through the control electrode 2 and across the co-axial line resonator 3 to a reflecting electrode 2. The resonator 3 will usually be maintained at a positive potential with respect to the oathode, and electrodes 2 and A will be mainta ned at negative potentials. The magnetic field 1s m the same direction as the beams as indicated by the dotted lines and arrows.

The electrons leaving the middle of the oathode will be constrained to follow approximately the magnetic lines of force, which will keep them in the middle of the slot through the resonator. They will be reflected by the electrode 4 and will follow nearly the same path on their return. After this second transit it is obvious that many of them must have lost ener y, otherwise the oscillation of the resonator could not be maintained. A large'proportion of the electrons will therefore be unable to reach the cathode and will be reflected back again through the system. If, as is usual, the magnetic field is strong, they will never be able to reach the sides of the slots. (For example, in a practical case where the field strength was 1100 oersteds, the electron velocity perpendicular to the beam must be equivalent to the energy of 1'70 electron volts to enable them to reach the sides of the slot.) Outside the resonator 3 the electric field will not be wholly parallel to the beam, but will have small components transverse thereto. The effect of these small transverse components acting with the magnetic field is to deflect the electrons perpendicularly to the plane of the paper by small amounts, and the direction of displacement is likely to be different near the control electrode from that near the resonator. Eventually the successive small displacements will add up sufficiently to allow the electrons to reach the end of the slot and be collected, but it is clear that the number of transits made by some of the electrons before this happens may be very great. Fig. 2 shows a typical track of an electron gradually displaced downwards in this way.

Figs. 3 and 4 show how according to the invention, the electron may be collected after two transits, one in each direction.

The slot in the resonator opposite the cathode is closed by a strip 5 attached to the resonator and provided with a number of small apertures I spaced apart in a line conforming with the slot, in order, to divide the electron beam into a number of small separated pencils. The control electrode 2 should at the same time be modified to direct the electrons through the apertures so that as few electrons as possible are lost by being collected on the surfaces between the apertures. This control electrode might be a wire grid having a pitch equal to the distance between the apertures, or it might be substantially a duplicate of the strip 5, as shown in Fig. 3. In either case, the control electrode should preferably be placed close to the cathode and should be registered with the strip 5 as indicated in Fig. 3.

Between the resonator 3 and the reflecting electrode 4 are placed two strip electrodes 6, arranged parallel to the beam on either side. An electric field is maintained between these two electrodes, so that while passing between them, the'electrons are deflected parallel to the slot in the resonator under the influence of the combined magnetic and electric fields. as shown in Fig. 3, without being appreciably deflected s deways as viewed in Fig. 4. The amount of deflection parallel to the slot and its direction depend on the strength and sign of the electric field maintained between the strips 6, which field may be adjustedso that an electron having initially passed through one of the apertures in the strip 5 is reflected so as to strike the adjacent surface and be collected, as shown in Fig. 3. By this means the electrons will be prevented from making more than two transits, so that the cause of the excessive noise is removed. A further advantage of the system is that the returned electrons cannot penetrate into the space between the cathode and the resonator, and cannot, therefore, afiect the emission current. This was a source of difliculty in the devices of existing type as shown'in Figs. 1 and 2, in which the space charge in this region is found to be much affected by the reflected electrons, so that considerable change in the current occurs at the commencement of oscillation, sometimes leading to instability,

By comparing Figs. 2 and 3, it will be seen that even without the deflecting plates 6, the use of a divided beam will be of considerable advantage in reducing the number of transits before the electrons are finally collected, and this may be suificient to reduce the noise to an allowable level, provided the size and spacing of the apertures in the plate 5 are appropriately chosen.

Figs. 5 and 6 show in more detail the arrangements of the various parts in two embodiments of the invention. In Fig. 5, the control electrode 2 is in the form of a wire grid, the wires being spaced the same distance apart as the slots in the strip 5. The deflecting plates 6 are shown behind the resonator 3, and the reflector i is a plate bent to form three sides of a polygon. 8 is a screen to absorb electrons emitted backwards from the cathode, and 9 is the lead 1for the central conductor of the co-axial resona- Fig. 6 shows an arrangement in which the deflecting plates are omitted, and in which the control electrode is a plate perforated with apertures similarly to the plate 5. The reflecting electrode 4 is in this case a flat plate.

In a particular case of Fig. 6, satisfactory reduction of noise was obtained with an operating voltage on the resonator between and 500 volts when there were about 7 apertures in 2 and 5, each aperture being 0.04 inch wide as measured in the direction of the length of the slot, the spacing between the centres of the apertures being 0.065 inch, so that the overall width of the beam was about 0.7 inch. The strength of the magnetic field was about 1100 oersteds, and the device operated at wave-lengths of the order of 10 c. m.

It will be evident that either type of control electrode may be used in eitherof the devices shown in Figs. 5 and 6.

Fig. 7 shows an assembled structure of Fig. 5 within an envelope, l0. In this figure parts corresponding to those in Fig. 5 have been given the same reference numbers.

It may be pointed out that the device according to the invention described with reference to Figs. 3, 4 and 5 is one which allows the passage of the electrons in one direction, but which prevents the electrons travelling in the opposite direction from returning to the neighborhood of the cathode or control electrode.

What is claimed is: v

'1. An electron discharge apparatus of the electron velocity modulated type comprising a cavity resonator having an entrance aperture and an exit aperture in the wall thereof, said apertures being in alignment on opposite sides of the axis of said resonator, means adjacent said entrance aperture for producing and projecting a beam of electrons through said apertures,

' means adjacent said exit aperture for reflecting said beam of electrons back into said cavity resonator after emergence of said beam of electrons from said exit aperture, means for producing a field intermediate said cavity resonator and said reflecting means, for deflecting said beam of electrons after emergence thereof from said exit aperture, and means for collecting said beam of reflected electrons, positioned intermediate said exit aperture and said means for producing and projecting said beam of electrons, which means are out of alignment with said apertures.

2. An electron discharge apparatus according to claim 1, wherein said means for producing and projecting a beam of electrons comprises a cathode, said cathode being disposed adjacent to, and in alignment with, the apertures of said cavity resonator.

3. An electron discharge apparatus according to claim 1, wherein the means for producing and projecting a beam of electrons comprises a source of electrons disposed adjacent to, and in alignment with the apertures of said cavity resonator, and a control electrode, disposed intermediate said source of electrons and said cavity resonator.

4. An electron discharge apparatus according to claim 1, wherein the means for reflecting said beam of electrons comprises a reflector electrode, positioned opposite the exit aperture of said cavity resonator 5. An electron discharge apparatus according to claim 1 wherein the means for producing a field and for deflecting said beam of electrons comprises a pair of deflecting electrodes parallel to and spaced from each other and positioned intermediate said cavity resonator and said means for reflecting said beam of electrons, said deflecting electrodes also being positioned on opposite sides of the path of said beam of electrons.

6. An electron discharge apparatus according to claim 1, wherein said cavity resonator comprises a plate attached to the wall thereof, adjacent said means for producing and projecting a beam of electrons, said plate having at least one aperture therein, said aperture constituting the entrance aperture of said cavity resonator, and another portion of said plate constituting the means for collecting said beam of reflected electrons.

'7. An electron discharge apparatus of the electron velocity modulated type comprising a cavity resonator having a plate attached to the wall thereof, said plate having a plurality of entrance apertures therein, which apertures are in alignment with an exit aperture on the opposite side 6 ity of apertures forming means for collecting said beam of reflected electrons.

8. An electron discharge apparatus according to claim 7, wherein said means for producing and projecting a beam of electrons comprises a cathode disposed adjacent to, and in alignment with, the plurality of entrance apertures of said plate.

9. An electron discharge apparatus according to claim 7, wherein said means for producing and projecting a beam of electrons comprises a source of electrons, a control electrode disposed'intermediate said source of electrons and said cavity resonator, said control electrode having a plurality of apertures therein, which apertures are in register with said entrance apertures.

10. An electron discharge apparatus according to claim 1, wherein said cavity resonator comprises a co-axial line resonator, including an outer member having an entrance aperture and an exit aperture in the wall thereof, said apertures being in diametric alignment, and an inner member having a passage therethrough in alignment with said entrance and said exit apertures.

11. An electron discharge apparatus according to claim 1, wherein said cavity resonator comprises a co-axial line resonator, including an outer member having an entrance aperture and an exit aperture in the wall thereof, said apertures being in diametric alignment and an inner member having a passage therethrough in alignment with said entrance aperture and said exit aperture, and wherein said means for reflecting said beam of electrons comprises a reflector electrode, and said means for producin a field and for deflecting said beam of electrons, comprises a pair of deflecting electrodes parallel to and spaced from each other, and positioned intermediate said coaxial line resonator and said reflector electrode, said deflecting electrodes also being positioned on opposite sides of the path of said beam of electrons.

12. An electron discharge apparatus according to claim 1, wherein said cavity resonator comprises a co-axial line resonator, including an outer member having a plurality of entrance apertures and an exit aperture in the wall thereof, said entrance apertures being in diametric alignment with said exit aperture, and an inner member having a passage therethrough in alignment with said entrance and exit apertures.

13. An electron discharge apparatus according to claim 1, wherein said cavity resonator comprises a co-axial line resonator, includin an outer member having a plurality of entrance apertures and an exit aperture in the wall thereof, said entrance apertures being in diametric alignment with said exit aperture, and an inner member having a passage therethrough in alignment with said entrance and said exit apertures, and wherein the means for producing and projecting a beam of electrons comprises a cathode disposed adjacent to and in alignment with said apertures of said co-axial line resonator, and a control electrode disposed intermediate said cathode and said coaxial line resonator, said control electrode having a plurality of apertures therein, said apertures being in register with said entrance apertures.

14. An electron discharge apparatus of the electron velocity modulated type comprising a cavity resonator having apertures therein, means adjacent an aperture in said cavity resonator and external thereto, for producing and projecting a beam of electrons along a first path through said cavity resonator and a pair of said apertures, means adjacent the aperture in said cavity reso- 8 n'ator from which said beam of electronsemerges, for deflecting and reflecting said beam of elec- REFERENCES CITED tron along a second path extending i t i The following references are of record in the cavity resonator and through an aperture therein, file of this P t: means for collecting said reflecting beam of eiec- 5 UNITED STATES PATENTS trons, which means are positioned mtermediate said last mentioned aperture and said means for Number Name t producing and projecting said beam of electrons. e- 22,506 Hahn June 27, 1944 2,075,379 Varian Mar. 30, 1937 JOHN HEAVER FREMLIN. 10 9 Seiier Aug. 22, 1939 2,250,511 Varian et a1 July 29, 1941 2,320,860 Fremlin June 1, 1943 

